Disc storage devices are used in data processing systems for storing relatively large amounts of information that can generally be accessed in milliseconds. Structurally, a typical storage device comprises a rotating magnetizable disc medium having several surfaces, in the form of an assembly of one or more stacked platters, on which data is magnetically sensed and/or recorded in addressable sectors located on circular data track centerlines. The disc assembly is mounted on a drive spindle in the storage device that rotates at a constant high speed. The storage device also includes one or more transducers or read/write heads, associated with each surface of the disc. The transducers are mounted in spaced relation on an arm of a movable transducer carriage. The servo controller actuates the carriage in a controlled fashion to move all the data heads in unison radially over the disc surfaces to position any one of the data heads over a selected track centerline. Since all the data heads on the carriage move together, the device also includes control circuitry that selects one of the read/write heads to perform a data transfer operation.
The servo controller responds to commands from the data processing system. The controller does this by transforming those commands into an analog servo signal which ultimately drives, usually through a power amplifier, an electromechanical actuator that connects to the transducer carriage. Typically, the disc device operates in one of two modes. The first (usually open-loop) is a mode in which the carriage, and thus the selected data head, is driven to the vicinity of the desired circular track centerline. Once that data head reaches that vicinity, the system is switched to a track following (or closed-loop, servo controlled) mode. In the track following mode, the position of the actuator or carriage is servo controlled to cause the center of the selected data head to align itself with the centerline of the data track.
To minimize alignment error, servo systems typically employ formatting information prerecorded on the data disc to allow the controller to detect the displacement between the data head and the track centerline. A format might include servo data that is continuously prerecorded along servo tracks on a dedicated surface of the disc assembly (dedicated servo data) together with servo data prerecorded in circumferentially spaced servo sectors interspersed, or embedded between adjacent pairs of storage data sectors on a data surface of the disc assembly (embedded servo data). Dedicated servo data is typically read by a read-only servo head, while embedded servo data is read along with the data by a read/write head, and thereafter separated from the data by servo data processing circuitry.
The servo data from both the dedicated and data surfaces is decoded by the disc controller, thereby enabling it to modify a servo control signal, if necessary, and thus continuously maintain the position of the data head in alignment with the selected data track centerline. However, several factors limit the alignment accuracy and thus, the maximum attainable data track density, of a disc storage device. The most common of these factors stem from electrical and mechanical disturbances, or noise. DC bias forces and electrical offsets are examples of some disturbances. A notable mechanical disturbance is spindle runout, or wobble, which is the difference between the actual centerline of a track and the effective centerline presented to a head positioned a fixed distance from the mounting center of the disc. It is typically caused by slight eccentricity in the mounting of the disc on a drive spindle. Runout occurs in disc systems using exchangeable disc cartridges and results from even the slightest off-center mounting (e.g., a fraction of a thousandth of an inch) as well as from slippage or tilt in seating of the disc cartridge after mounting. Carriage play between the transducer carriage and its guide rods, as well as disalignment due to uneven thermal expansion of the carriage, arms, disc or transducers further contribute to the mechanical disturbances. Generally, positioning tolerances should be within .+-.10% maximum of track pitch, i.e., the spacing between adjacent track centerlines). Thus, for example, a 1000 track-per-inch servo system should maintain a data head within .+-.100 microinches of a data track centerline. With typical currently available exchangeable disc systems, such alignment accuracy is not easily obtainable.
Control system lag is another factor that effects positioning accuracy. Lag is the time delay between the time the controller detects an off-track condition and the time the actuator begins to move the transducer into alignment with the data track centerline. Some of this delay is attributable to the electrical response characteristics of the servo control system, as, for example, resulting from a low sampling rate; the remaining delay is attributable to the mechanical response characteristics of the electromechanical actuator. Such delays characterize the band width of the servo control system. The greater the band width, the faster the positioning system can respond to an off-track condition, thereby providing tightly controlled positioning of the data head. A positioning system having high band width provides increased data track density because centerlines can be followed within a smaller tolerance. There are other factors that contribute to misalignment during track following operations.
Conventional methods of increasing servo band width include increasing the frequency of structural mechanical resonances, providing continuous position feedback from a dedicated servo surface, and providing a higher sample rate position feedback emanating from the data surfaces.
An approach for overcoming some of the effects of the electrical and mechanical disturbances has been to improve the tolerances of the mechanical and electrical circuit components of the servo system. However, this is an expensive proposition and is only marginal at best in solving the problems. Servo compensation networks have also been used to reduce head misalignment resulting from uneven theremally induced dimension or position changes of the mechanical components. This approach only partially corrects misalignment errors of the transducer because it is based on a model that attempts to correct only some of the average offset errors, but not the runout errors.
A number of approaches have been disclosed for improving head alignment electronically. One approach has been to provide sectorized, or embedded, servo positioning data on the data storage track, as an alternative to or in supplement to servo positioning information on a dedicated disc surface. However, this approach does not overcome band width limitations. Another approach has been to use a transducer positioned at a radially fixed stationary reference point over a position reference track on the rotating disc to detect misposition error signals. However, this also does not provide an optimum result, because it lacks face compensation, noise reduction, or a close fixed relationship to the pecularities of positioning of the actual track being read.
Another approach has been to derive misposition error signal from a course positioner on the transducer carriage rather than the disc medium. Again, noise reduction and iteration are lacking in such a system.
Another approach has been to provide first and second sets of servo signals on a data surface recorded in alternate track locations at centerlines shifted radially by the width of one half track with respect to the centerlines of the storage data tracks. However, the approach to using this information is to make several passes over the information and store a set of misposition error signals so that the correction signals can be utilized during subsequent read/write operations. This system must use considerable processing hardware and memory space for storing the information. Moreover, it is incapable of dynamically correcting for modifications that may occur during the use of the system.
The other critical limitation in accurate positioning systems is the arrangement and sequence of energization of the motor coils. Although motors with multiple phases have become well know to produce greater numbers of steps with high torque, the critical feature now has become the control circuitry to switch the currents to the coils. Such circuitry is typically quite complex, and is incapable of providing the desired number of steps per electrical revolution.